Method for producing mgb2 superconductor

ABSTRACT

A alloy (Mg—X) of metal (X) and Mg in a liquid phase is made to react with B in a solid phase at a low temperature to manufacture a superconductor, which contains a large amount of MgB 2  potential for MRI, linear motorcar, superconducting cavity, electric power transmission cable, high-magnetic field magnet for medical units, electric power storage (SMES), and the like and is formed in the shape of bulk, wire, and foil, by heat treatment performed at a low temperature for a short time and at low cost.

TECHNICAL FIELD

The invention of this application relates to a method of manufacturing aMgB₂ superconductor. More specifically, the invention of thisapplication relates to a new method of manufacturing a superconductorthat is useful for a MRI, a linear motorcar, a superconducting cavity,an electric power transmission cable, a high-magnetic field magnet formedical units, an electric power storage (SMES), and the like andcontains MgB₂ to be used at the highest temperature among metal-basedsuperconductors as a main component and is formed in the shape of bulk,wire, foil, and the like by heat treatment performed at a lowtemperature and for a short time.

BACKGROUND ART

The conventionally known superconductors are broadly divided into acopper oxide superconductor made of copper oxide and a non-copper oxidesuperconductor made of material other than copper oxide. Among them, thecopper oxide superconductor has a higher superconducting dislocationtemperature (Tc) than the non-copper oxide superconductor. However, thecopper oxide superconductor has a drawback that it is hard to workbecause it is ceramic made of copper and oxygen and having the so-calledtwo-dimensional surface structure. Further, on the other hand, anintermetallic compound made of only a plurality of metal elements suchas MgB₂ has been known as a non-copper oxide superconductor (patentdocument 1).

[Patent document 1] Japanese Patent Application Laid-Open No.2002-211916

This superconductor made of MgB₂ is comparatively easily formed andworked as compared with the copper oxide superconductor. However, eventhe intermetallic compound of MgB₂ that is comparatively easily formedand worked as compared with the copper oxide superconductor has thefollowing drawbacks: in a conventional manufacturing method of making Mgof a liquid phase react with B of a solid phase or making Mg of a gasphase react with B of a solid phase, heat treatment performed at a hightemperature for a long time or a condition of high pressure is requiredand hence the manufacturing of the MgB₂ superconductor is not practical;and in a method of using the solid phase diffusion reaction of Mg of asolid phase and B of a solid phase, the rate of formation is extremelyslow.

Further, the superconductor manufactured by any one of the conventionalmanufacturing methods has not been always satisfactory in practicalperformance such as Jc characteristic and strain-resistancecharacteristic.

Hence, the object of the invention of this application is to provide anew method of manufacturing a MgB₂ superconductor by which asuperconductor, whose main component is MgB₂ excellent in theabove-described practical characteristics, can be manufactured easilyand efficiently by heat treatment performed at or in the vicinity of anormal pressure and at a lower temperature and for a short time.

DISCLOSURE OF THE INVENTION

In order to solve the above problems, firstly, the invention of thisapplication provides a A method of manufacturing a MgB₂ superconductorwhich comprises making a Mg—X alloy of an alloy of Mg and an element Xforming an alloy having a lower melting point than the melting point ofMg react with B through diffusional reaction at a temperature of 800° C.or lower to manufacture a superconductor having MgB₂ as a maincomponent. Secondly, the invention provides a method of manufacturing aMgB₂ superconductor, wherein the Mg—X alloy is made to react with Bthrough diffusional reaction at a temperature range from 400° C. to 800°C. Thirdly, the invention provided a method of manufacturing a MgB₂superconductor, wherein the Mg—X alloy is made to react with B throughdiffusional reaction at a temperature in the vicinity of 650° C. for 4hours or more. Fourthly, the invention provides a method ofmanufacturing a MgB₂ superconductor, wherein the Mg—X alloy is made toreact with B through diffusional reaction in a vacuum or in an inert gasatmosphere.

Then, as for the above-described method, fifthly, the invention of thisapplication provides a method of manufacturing a MgB₂ superconductor,wherein the element X is one element or more selected from a groupconsisting of Ag, Cu, Sn, Ga, Pb, In, Bi, and Zn.

Further, the invention of this application, sixthly, provides a methodof manufacturing a MgB₂ superconductor, wherein a mixture of the Mg—Xalloy and B is worked or overlaid on a base material and then issubjected to heat treatment. Seventhly, the invention provides a methodof manufacturing a MgB₂ superconductor, wherein the mixture of the Mg—Xalloy and B is crammed into a metal pipe to be plastically deformed andis drawn into a wire and that the wire is then subjected to heattreatment. Eighthly, the invention provides a method of manufacturing aMgB₂ superconductor wherein the mixture of the Mg—X alloy and B iscrammed into a metal pipe to be plastically deformed and is drawn into awire and that the wire is then subjected to heat treatment to form asingle filament wire and that a large number of obtained single filamentwires are crammed into the same metal pipe and are drawn into a wire toform an extremely-fine multifilament wire and that the extremely-finemultifilament wire is subjected to heat treatment. Ninthly, theinvention provides a method of manufacturing a MgB₂ superconductor,wherein the mixture of the Mg—X alloy and B is dispersed in an organicsolvent to produce a solution and that the solution is applied to aheat-resistant substrate and then is subjected to heat treatment.Tenthly, the invention provided a method of manufacturing a MgB₂superconductor, wherein wire-shaped B is heated and is passed through abath of the Mg—X alloy brought into a molten state previously at itsheating temperature to overlay the Mg—X alloy on a surface of thewire-shaped B and then is subjected to heat treatment.

Still further, eleventhly, the invention provides a method ofmanufacturing a MgB₂ superconductor wherein X in the Mg—X alloy is 50 at% or less C for Mg. Twelfthly, the invention provides a method ofmanufacturing a MgB₂ superconductor, wherein X in the Mg—X alloy is 35at % or less Ag for Mg. Then, thirteenthly, the invention provides amethod of manufacturing a MgB₂ superconductor, wherein X in the Mg—Xalloy is 25 at % or less or from 50 at % to 95 at % Sn for Mg. Further,fourteenthly, the invention provides a method of manufacturing a MgB₂superconductor, wherein X in the Mg—X alloy is 95 at % or less Ga forMg. Fifteenthly, the invention provides a method of manufacturing a MgB₂superconductor wherein X in the Mg—X alloy is 95 at % or less Pb for Mg.Sixteenthly, the invention provides a method of manufacturing a MgB₂superconductor wherein X in the Mg—X alloy is 95 at % or less In for Mg.Seventeenthly, the invention provides a method of manufacturing a MgB₂superconductor characterized in that X in the Mg—X alloy is 30 at % orless or from 45 at % to 95 at % Bi for Mg. Eighteenthly, the inventionprovides a method of manufacturing a MgB₂ superconductor wherein X inthe Mg—X alloy is 95 at % or less Zn for Mg.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph to show a change in Ic caused by a tensile strain of aMgB₂ wire in the comparison between (a) and (b):

(a) is a MgB₂ wire manufactured by the reaction of Mg and B,

(b) is a MgB₂ wire manufactured by the reaction of Mg—Cu alloy and B.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention of this application has the above-described features andthe embodiments of the invention will be described below.

According to the invention of this application, a Mg—X alloy made of Mgand a third element of X and having a low melting point is previouslyformed before Mg (magnesium) is made to react directly with B (boron),and the formed Mg—X alloy having a low melting point is heated at a lowtemperature, thereby being reduced to a liquid phase, and then the Mg—Xalloy reduced to the liquid phase is made to react with B by diffusion.

In the invention of this application, as for the requirements of thethird element (X) forming an alloy with Mg, the melting point of theMg—X alloy to be formed needs to be lower than the melting point (650°C.) of Mg. Further, it is desired that when X reacts with B, atwo-dimensional compound such as X—B or a three-dimensional compoundsuch as X—Mg—B is not formed. As the third element (X) suitablysatisfying this requirements is considered one or more kind of Ag(silver), Cu (copper), Sn (tin), Ga (gallium), Pb (lead), In (indium),Bi (bismuth), and Zn (zinc).

By utilizing the specific third element (X) that reacts with Mg to forman alloy to lower the melting point of Mg in this manner, the inventionof this application makes it possible to manufacture a superconductor,which has MgB₂ excellent in the practical characteristics (Jccharacteristic, strain-resistance characteristic) as a main componentand is formed in the shape of bulk, wire, or foil, only by performingheat treatment at a low temperature for a short time under a reducedpressure, a normal pressure, or the application of pressure.

The physical properties of the MgB₂ superconductor manufactured by theinvention of this application differ depending on the alloy ratio of thethird element (X) to Mg and the reaction temperature when the formedMg—X alloy is made to react with B, and the mode of the invention willbe described in detail in the embodiments. Further, the mode of methodof manufacturing a superconductor formed in the shape of bulk, wire, andfoil will be described also in the embodiments.

When Cu (copper) or Ag (silver) is used as the third element (X), aMg—Cu alloy or a Mg—Ag alloy are formed and when this Mg—Cu alloy orMg—Ag alloy is made to react with B to manufacture MgB₂, as a byproduct,a Cu-based dilute alloy phase or a Ag-based dilute alloy phase is formedas the second phase. The Cu-based dilute alloy phase or the Ag-baseddilute alloy phase formed as the second phase plays a role in preventingthe occurrence of cracks in the MgB₂ phase even when a strain is appliedto the MgB₂ superconductor, thereby improving brittleness. Hence, evenif a high strain occurs in the MgB₂ superconductor, the MgB₂superconductor is not degraded in superconducting characteristics andhence can be easily handled.

Further, when a third element (X) is selected that forms the secondphase forming a Cu-based dilute alloy phase or a Ag-based dilute alloyphase excellent in electric conductivity, even if a superconductingstate is locally broken, an electric current bypasses the locally brokenportion and passes the Cu-based dilute alloy phase or the Ag-baseddilute alloy of the second phase to prevent heating, which produces aneffect of stabilizing the superconducting state.

In addition, the invention of this application can provide asuperconductor having an extremely large Jc at low cost to reduce theamount of use of the superconductor. Further, the formed second phasehas an action to strengthen the superconductor also mechanically andhence the superconductor is excellent in resistance to an external forcesuch as electromagnetic force or the like.

Hence, the embodiments will be shown and described in more detail. Theinvention is not limited by the following embodiments. Embodiment

Embodiment 1

An alloy powder of Mg-33 at % Cu was mixed with a B powder at a molratio of Mg:B=1:2 and the mixed alloy powder was pressed and then wassubjected to heat treatment in a vacuum at 450° C. for 500 hours, at550° C. for 100 hours, at 600° C. for 4 hours, and at 750° C. for 0.5hours, which revealed that there were produced superconductors having aTc of 23K, 38.5 K, 39.1 K, 39.3 K, and 21 K. A clear diffraction patternof MgB₂ was obtained by the X-ray diffraction of this substance. It wasrecognized from this diffraction pattern that a bulk-shaped MgB₂superconductor was obtained. In this regard, it was recognized by EDAXobservation that a Cu-based dilute alloy phase was formed in the samplein addition to the MgB₂ phase. Further, even when the ratio of Cu in thealloy with Mg was increased to 50 at %, an excellent superconductingsubstance was obtained.

Further, while the above-described heat treatment was performed in thevacuum, it was recognized that the heat treatment could be performed inan inert gas atmosphere such as argon or nitrogen.

On the other hand, when an alloy powder of Mg-55 at % Cu was used, itwas not easy to form MgB₂ at a heating temperature lower than 800° C.,and although MgB₂ could be formed at a heating temperature higher than800° C., superconducting characteristics were not practicallysufficient.

Embodiment 2

An alloy powder of Mg-20 at % Ag was mixed with a B powder at a molratio of Mg:B=1:2 and the mixed powder was pressed and then wassubjected to heat treatment in a vacuum at 450° C. for 500 hours, at550° C. for 100 hours, at 600° C. for 10 hours, at 650° C. for 4 hours,and at 750° C. for 1.5 hours, which revealed that there were producedsuperconductors having a Tc of 22.1K, 34.7 K, 39.2 K, 39.3 K, and 19.9K. A clear diffraction pattern of MgB₂ was obtained by the X-raydiffraction of this substance. In this regard, it was recognized by EDAXobservation that a Ag-based dilute alloy phase was formed in the samplein addition to the MgB₂ phase. Further, even when the ratio of Ag in thealloy with Mg was increased to 35 at %, an excellent superconductingsubstance was obtained.

On the other hand, when an alloy powder of Mg-40 at % Ag was used, apractically satisfactory superconductor was not always obtained.

Embodiment 3

An alloy powder of Mg-15 at % Sn was mixed with a B powder at a molratio of Mg:B=1:2 and the mixed alloy powder was pressed and then wassubjected to heat treatment in a vacuum at 550° C. for 100 hours, at600° C. for 10 hours, and at 650° C. for 4 hours, which revealed thatthere were produced superconductors having a Tc of 31.7 K, 38.8 K, and39.3 K. A clear diffraction pattern of MgB₂ was obtained by the X-raydiffraction of this substance. In this regard, it was recognized by EDAXobservation that a Sn-based dilute alloy phase was formed in the samplein addition to the MgB₂ phase.

Further, an alloy powder of Mg-60 at % Sn was mixed with a B powder at amol ratio of Mg:B=1:2 and the mixed alloy powder was pressed and thenwas subjected to heat treatment in a vacuum at 550° C. for 100 hours, at600° C. for 10 hours, and at 650° C. for 4 hours, which revealed thatthere were produced superconductors having a Tc of 30.3 K, 36.5 K, and39.2 K. A clear diffraction pattern of MgB₂ was obtained by the X-raydiffraction of this substance. In this regard, it was recognized by EDAXobservation that a Sn-based dilute alloy phase was formed in the samplein addition to the MgB₂ phase.

On the other hand, when an alloy powder of Mg-33 at % Sn was mixed witha B powder at a mol ratio of Mg:B=1:2 and the mixed alloy powder waspressed and then was subjected to heat treatment in a vacuum at atemperature higher than 800° C., it was recognized that a small amountof MgB₂ was formed but a substance having excellent superconductingcharacteristics was not obtained.

Further, when an alloy powder of Mg-96 at % Sn was mixed with a B powderat a mol ratio of Mg:B=1:2 and the mixed alloy powder was pressed andthen was subjected to heat treatment in a vacuum at 550° C. for 100hours, at 600° C. for 10 hours, and at 650° C. for 4 hours, it wasclearly recognized that MgB₂ was formed in the sample but asuperconducting current density per whole cross-sectional area of thesample, that is, an overall Jc (4.2 K, 3T) was 20 A/cm², which was notsufficiently satisfactory from a practical point of view.

From the above results, it was recognized that in the alloy of Sn andMg, an excellent superconducting substance was obtained within a rangewhere Sn is 25 at % or less or from 50 at % to 95 at %.

Embodiment 4

An alloy powder of Mg-28 at % Ga was mixed with a B powder at a molratio of Mg:B=1:2 and the mixed alloy powder was pressed and then wassubjected to heat treatment in a vacuum at 550° C. for 100 hours, at600° C. for 10 hours, and at 650° C. for 4 hours, which revealed thatthere were produced superconductors having a Tc of 29.7 K, 37.7 K, and39.2 K. A clear diffraction pattern of MgB₂ was obtained by the X-raydiffraction of this substance. In this regard, it was recognized by EDAXobservation that a Ga-based dilute alloy phase was formed in the samplein addition to the MgB₂ phase.

On the other hand, an alloy of Mg-96 at % Ga having a melting point nearroom temperature was brought into a molten state at 200° C. and wasmixed with a B powder at a mol ratio of Mg:B=1:2 and the mixed substancewas subjected to heat treatment in a vacuum at 550° C. for 100 hours, at600° C. for 10 hours, and at 650° C. for 4 hours. Then, it was clearlyrecognized that MgB₂ was formed in a liquid phase of Ga but that theamount of MgB₂ was not necessarily large. Hence, from a practical pointof view, it was recognized that it was preferable to utilize an alloyhaving a Ga concentration of 95 at % or less.

Embodiment 5

An alloy powder of Mg-33 at % Pb was mixed with a B powder at a molratio of Mg:B=1:2 and the mixed alloy powder was pressed and then wassubjected to heat treatment in a vacuum at 550° C. for 100 hours, at600° C. for 10 hours, and at 650° C. for 4 hours, which revealed thatthere were produced superconductors having a Tc of 28.6K, 38.3 K, and39.1 K. A clear diffraction pattern of MgB₂ was obtained by the X-raydiffraction of this substance. In this regard, it was recognized by EDAXobservation that a Pb-based dilute alloy phase was formed in the samplein addition to the MgB₂ phase. Further, it was recognized that even whenthe ratio of Pb in the alloy with Mg was increased to 95 at %, anexcellent superconducting substance was obtained.

On the other hand, when an alloy powder of Mg-96 at % Pb was mixed witha B powder at a mol ratio of Mg:B=1:2 and the mixed alloy powder waspressed and then was subjected to heat treatment in a vacuum at 550° C.for 100 hours, at 600° C. for 10 hours, and at 650° C. for 4 hours, itwas recognized that MgB₂ was formed in all samples but a superconductingcurrent density per whole cross-sectional area of the sample, that is,an overall Jc (4.2 K, 3T) was 16 A/cm², which was not necessarilypractical.

Embodiment 6

An alloy powder of Mg-20 at % In was mixed with a B powder at a molratio of Mg:B=1:2 and the mixed alloy powder was pressed and then wassubjected to heat treatment in a vacuum at 550° C. for 100 hours, at600° C. for 10 hours, and at 650° C. for 4 hours, which revealed thatthere were produced superconductors having a Tc of 28.6 K, 38.3 K, and39.1 K. A clear diffraction pattern of MgB₂ was obtained by the X-raydiffraction of this substance. In this regard, it was recognized by EDAXobservation that an In-based dilute alloy phase was formed in the samplein addition to the MgB₂ phase.

Further, it was recognized that MgB₂ was formed even in the case ofMg-96 at % In. However, it was recognized that since the amount of MgB₂was small, from a practical point of view, it was preferable to make theratio of In smaller than 95 at %.

Embodiment 7

An alloy powder of Mg-90 at % Zn was mixed with a B powder at a molratio of Mg:B=1:2 and the mixed alloy powder was pressed and then wassubjected to heat treatment in a vacuum at 550° C. for 100 hours, at600° C. for 10 hours, and at 650° C. for 4 hours, which revealed thatthere were produced superconductors having a Tc of 23.5 K, 28.4 K, and32.8 K. A clear diffraction pattern of MgB₂ was obtained by the X-raydiffraction of this substance. In this regard, it was recognized by EDAXobservation that a Zn-based dilute alloy phase was formed in the samplein addition to the MgB₂ phase.

Further, it was recognized that MgB₂ was formed even in the case ofMg-96 at % Zn. However, it was recognized that since the amount of MgB₂was small, from a practical point of view, it was preferable to make theratio of Zn smaller than 95 at %.

Embodiment 8

An alloy powder of Mg-25 at % Bi was mixed with a B powder at a molratio of Mg:B=1:2 and the mixed alloy powder was pressed and then wassubjected to heat treatment in a vacuum at 550° C. for 100 hours, at600° C. for 10 hours, and at 650° C. for 4 hours, which revealed thatthere were produced superconductors having a Tc of 30.4 K, 32.1 K, and36.8 K. A clear diffraction pattern of MgB₂ was obtained by the X-raydiffraction of this substance. In this regard, it was recognized by EDAXobservation that a Bi-based dilute alloy phase was formed in the samplein addition to the MgB₂ phase.

Further, an alloy powder of Mg-60 at % Bi was mixed with a B powder at amol ratio of Mg:B=1:2 and the mixed alloy powder was pressed and thenwas subjected to heat treatment in a vacuum at 550° C. for 100 hours, at600° C. for 10 hours, and at 650° C. for 4 hours, which revealed thatthere were produced superconductors having a Tc of 30.3 K, 36.5 K, and39.2 K. A clear diffraction pattern of MgB₂ was obtained by the X-raydiffraction of this substance. In this regard, it was recognized by EDAXobservation that a Bi-based dilute alloy phase was formed in the samplein addition to the MgB₂ phase.

On the other hand, an alloy powder of Mg-33 at % Bi was mixed with a Bpowder at a mol ratio of Mg:B=1:2 and the mixed alloy powder was pressedand then was subjected to heat treatment in a vacuum at temperatureslower than 800° C. and higher than 800° C. As a result, at a temperaturelower than 800° C., it was not recognized that marked MgB₂ was formedand at a temperature higher than 800° C., it was recognized that a smallamount of MgB₂ was formed but a substance having excellentsuperconducting characteristics was not obtained.

Further, an alloy powder of Mg-96 at % Bi was mixed with a B powder at amol ratio of Mg:B=1:2 and the mixed alloy powder was pressed and thenwas subjected to heat treatment in a vacuum in a vacuum at 550° C. for100 hours, at 600° C. for 10 hours, and at 650° C. for 4 hours. It wasclearly recognized that MgB₂ was formed in the sample but it turned outthat a superconducting current density per the whole cross-sectionalarea of the sample was not suitable for practical use.

From the above results, it turned out that a desirable superconductingsubstance can be obtained when Bi is 30 at % or less or from 45 at % to95 at %.

Embodiment 9

An alloy powder of Mg-33 at % Cu was mixed with a B powder at a molratio of Mg:B=1:2 and the mixed alloy powder was crammed into astainless pipe and then was drawn into a wire and then was subjected toheat treatment in a vacuum at 550° C. for 100 hours, at 600° C. for 10hours, and at 650° C. for 4 hours, whereby superconductors having a Tcof 38.5 K, 39.1 K, and 39.3 K were obtained. Further, values of 950A/mm², 1120 A/mm², and 1230 A/mm² were obtained as overallsuperconducting current densities Jc under conditions of 4.2 K and 3T.

When taking it into consideration that an overall Jc under usagemagnetic field and temperature of a superconductor at a practical stateis 200 A/mm² or more, the value at the commercially practical stage asthe value of Jc under conditions of 4.2 K and 3T was obtained.

Further, as is clear from the above embodiments 1 to 8, even if thethird element (X) is any one of them, in the case where the ratio of thethird element (X) to Mg is the specific value, when the third element(X) is made to react with Mg with a reaction temperature set at or inthe vicinity of 650° C. or more and for 4 hours or more, in most cases,a MgB₂ superconductor having excellent physical properties can beobtained.

Embodiment 10

An alloy powder of Mg-33 at % Pb was mixed with a B powder at a molratio of Mg:B=1:2 and the mixed alloy powder was crammed into astainless pipe and then was drawn into a wire and then was subjected toheat treatment in a vacuum at 650° C. for 4 hours to manufacture wires.

Then, a tensile strain was applied to each of a MgB₂ wire (a) made of analloy powder of Mg-33 at % Pb and a B powder and a MgB₂ wire (b) made ofan alloy powder of Mg-33 at % Cu and a B powder and the deterioration ofIc was measured and the measurement results are shown in FIG. 1.

As is clear from FIG. 1, in the MgB₂ wire (b) not containing Cu, thedeterioration of Ic starts at a strain of 0.4% whereas in the MgB₂ wire(a) containing Cu, the deterioration of Ic does not occur until strainbecomes larger than 0.8%. It can be thought that this is because theCu-based dilute alloy of the second phase can produce an effect ofpreventing the occurrence of cracks in MgB₂. This means that the MgB₂wire (a) containing Cu can be practically used for wires used in thestate of large tensile strain.

Embodiment 11

An alloy powder of Mg-33 at % Cu was mixed with a B powder at a molratio of Mg:B=1:2 and the mixed alloy powder was crammed into a copperpipe and then was drawn into a wire and then was subjected to heattreatment in a vacuum at 550° C. for 100 hours, at 600° C. for 10 hours,and at 650° C. for 4 hours, whereby superconductors each having anoverall Jc of 960 A/mm², 1180 A/mm², and 1310 A/mm² under conditions of4.2 K and 1 T were obtained. It turned out that a transition from asuperconducting state to a normal conducting state was moderate and thata branch flow to the copper pipe was produced to make a superconductingstate stable. Then, the 120 single wires obtained in this manner werebundled and crammed into a copper pipe again and then were drawn into awire and then the drawn wire was subjected to heat treatment in a vacuumat 550° C. for 100 hours, at 600° C. for 10 hours, and at 650° C. for 4hours. The obtained extremely-fine multifilament wire was furtherimproved in superconducting characteristics, that is, when themeasurement of a magnetization curve was made on the wire, a jump inmagnetic susceptibility, which is thought to be associated with a fluxjump, was not observed at all.

Embodiment 12

An alloy powder of Mg-66 at % Cu was mixed with a B powder at a molratio of Mg:B=1:2 and the mixed alloy powder was crammed into a silverpipe and then was drawn into a wire and then was subjected to heattreatment in a vacuum at 650° C. for 10 hours. Then, the same tensilestrain as in embodiment 10 was applied to the wire and the deteriorationof Ic was measured. As a result, a stabilized wire showing the samephysical properties as the MgB₂ wire (b) in embodiment 10 was obtained.

Embodiment 13

A B filament is passed through a furnace at 500° C., thereby beingpreviously heated, and then the heated B filament was passed through abath of alloy of Mg-60 at % In in a molten state at 500° C. at a speedof 1 m/sec to cover the surface of the B filament with the alloy ofMg-60 at % In. Then, this wire is subjected to heat treatment at 650° C.for 10 hours to manufacture MgB₂ on the surface of the B filament. Itwas recognized that this wire had a Tc of 39.1 K and an overall Jc of980 A/mm² under conditions of 4.2 K and 3 T. With this, it was shownthat a wire-shaped superconductor could be manufactured by a methodother than a wire drawing method.

Embodiment 14

An alloy powder of Mg-33 at % Cu was mixed with a B powder at a molratio of Mg:B=1:2 and the mixed alloy powder was dispersed in an organicsolvent (trichloroethylene) in the state of sol to make a solution.Then, the solution was applied to the surface of an iron-based substrateand was dried and then was subjected to heat treatment in a vacuum at650° C. for 10 hours to make a film (foil). The film (foil) had a Tc of39.1 K and an overall Jc of 300 A/mm² under conditions of 4.2 K and 3T.Further, these values are high values showing that the film can becommercially used as a superconducting thin film.

Embodiment 15

An alloy powder of Mg-33 at % Cu was mixed with a B powder at a molratio of Mg:B=1:2 and the mixed alloy powder was pressed and subjectedto heat treatment in an argon atmosphere under one atmospheric pressure.At a temperature lower than 400° C., it was not recognized that markedMgB₂ was formed and at a temperature higher than 800° C., it wasrecognized that Mg was markedly evaporated to form porous MgB₂-On theother hand, within a temperature range from 400° C. to 800° C., asuperconductor having a physical property similar to MgB₂ having a highTc and obtained in the embodiment 1 was manufactured. With this, it wasrecognized that the same reaction occurs not only in a vacuum but alsoin an inert gas.

INDUSTRIAL APPLICABILITY

According to the invention of this application, it is possible tomanufacture such a superconductor with ease and at low cost that issuitably used for MRI, linear motorcar, superconducting cavity, electricpower transmission cable, high-magnetic field magnet for medical units,electric power storage (SMES), and the like and contains MgB₂ as a maincomponent and is formed in the shape of bulk, wire, or foil. In thesuperconductor manufactured in this method, the second phase formed as abyproduct produces improvements in the stabilization of current andmechanical reinforcement.

1-18. (canceled)
 19. A method of manufacturing a MgB₂ superconductorwhich comprises making a Mg—X alloy of an alloy of Mg and either or bothof Ga and Bi as an element X forming an alloy having a lower meltingpoint than the melting point of Mg react with B through diffusionalreaction at a temperature of 800° C. or lower to manufacture asuperconductor having MgB₂ as a main component.
 20. The method ofmanufacturing a MgB₂ superconductor according to claim 19, wherein theMg—X alloy is made to react with B through diffusional reaction at atemperature range from 400° C. to 800° C.
 21. The method ofmanufacturing a MgB₂ superconductor according to claim 20, wherein theMg—X alloy is made to react with B through diffusional reaction at atemperature in the vicinity of 650° C. for 4 hours or more.
 22. Themethod of manufacturing a MgB₂ superconductor according to claim 19,wherein the Mg—X alloy is made to react with B through diffusionalreaction in a vacuum or in an inert gas atmosphere.
 23. The method ofmanufacturing a MgB₂ superconductor according to claim 19, wherein amixture of the Mg—X alloy and B is worked or overlaid on a base materialand then is subjected to heat treatment.
 24. The method of manufacturinga MgB₂ superconductor according to claim 23, wherein the mixture of theMg—X alloy and B is crammed into a plastic deformable metal pipe and isdrawn into a wire and then the wire is subjected to heat treatment. 25.The method of manufacturing a MgB₂ superconductor according to claim 23,wherein the mixture of the Mg—X alloy and B is crammed into a plasticdeformable metal pipe and is drawn into a wire and then the wire issubjected to heat treatment to form a single filament wire and a largenumber of obtained single filament wires are crammed into the same metalpipe and are drawn into a wire to form an extremely-fine multifilamentwire and the extremely-fine multifilament wire is subjected to heattreatment.
 26. The method of manufacturing a MgB₂ superconductoraccording to claim 23, wherein the mixture of the Mg—X alloy and B isdispersed in an organic solvent to produce a solution and the solutionis applied to a heat-resistant substrate and then is subjected to heattreatment.
 27. The method of manufacturing a MgB₂ superconductoraccording to claim 23, wherein wire-shaped B is heated and is passedthrough a bath of the Mg—X alloy brought into a molten state previouslyat the heating temperature to overlay the Mg—X alloy on a surface of thewire-shaped B and then is subjected to heat treatment.
 28. The method ofmanufacturing a MgB₂ superconductor according to claim 19, wherein X inthe Mg—X alloy is 95 at % or less of Ga based on Mg.
 29. The method ofmanufacturing a MgB₂ superconductor according to claim 19, wherein X inthe Mg—X alloy is 30 at % or less or from 45 at % to 95 at % of Bi basedon Mg.